The present invention relates to a process for the preparation of pure cyanuric acid from crude or waste melamine, which, in particular, contains ammeline and ammelide, using sulfuric acid.
Most of the processes for the preparation of cyanuric acid are based on the pyrolysis of urea, and a more or less highly contaminated end product is obtained which has to be purified before it can be put to further use.
Hitherto, only two processes have been disclosed which yield cyanuric acid from melamine or ammeline or ammelide as the starting material, using sulfuric acid.
U.S. Pat. No. 2,768,167 discloses a process for the preparation of cyanuric acid from melamine, ammeline, the ammeline:ammelide complex or mixtures thereof, according to which sulfuric acid maintained in a stoichiometric excess of up to 5% of free acid supplied as an aqueous solution thereof having a concentration of at least 5%, preferably 10 l to 20%, free acid is used as the mineral acid the hydrolysis is carried out at a temperature of at least 175.degree. C. under at least the autogenously developed pressure. Temperatures of 180.degree. to 200.degree. C. are preferred.
As has been found on repeating the Examples of U.S. Pat. No. 2,768,187, the said process can be carried out only when, as, say, in Example 1 of U.S. Pat. No. 2,768,187, the reaction is carried out with amounts of sulfuric acid which are substantially higher than those claimed. The molar ratio of melamine to sulfuric acid is 1:3.96 in the said Example. If, however, it is attempted to react melamine in an analogous manner to a mixture of ammeline and ammelide with 20% strength sulfuric acid by the procedure of Example 3 of U.S. Pat. No. 2,768,187, the molar ratio of sulfuric acid to the amino groups present to be 0.52, corresponding to the patent claims, the cyanuric acid obtained is not pure but, even after hydrolyzing for one hour instead of for the 10 to 20 minutes indicated, is only crude cyanuric acid containing about 15% of impurities.
According to a more recent process (Japan Kokai No. 1975, 32,193), the conversion of melamine, melamine-containing residues or melamine cyanurate to cyanuric acid is carried out with dilute mineral acid, for example sulfuric acid, and the reaction, although under normal pressure, must be carried out in two stages in order to obtain pure cyanuric acid. Ammeline salts are initially obtained in the first stage and these have to be further hydrolyzed to cyanuric acid in the second stage.
Surprisingly, it has now been found that cyanuric acid in a purity of more than 99% may be obtained from ammeline-containing and/or ammelide-containing crude or waste melamine by reaction with relatively highly concentrated sulfuric acid without adverse decomposition phenomena in a one-stage process under normal pressure.